This invention relates to cryogenic apparatus, and, more particularly, to cryogenic apparatus for making sensitive magnetic measurements.
The human body produces various kinds of energy that may be used to monitor the status and health of the body. Perhaps the best known of these types of energy is heat. Most healthy persons have a body temperature of about 98.6.degree. F. A measured body temperature that is significantly higher usually indicates the presence of an infection or other deviation from normal good health. A simple medical instrument, the clinical thermometer, has long been available to measure body temperature.
Over 100 years ago, medical researches learned that the body also produces electrical signals. Doctors today can recognize certain patterns of electrical signals that are indicative of good health, and other patterns that indicate disease or abnormality. The best known types of electrical signals are those from the heart and from the brain, and instruments have been developed that measure such signals. The electrocardiograph measures electrical signals associated with the operation of the heart, and the electroencephalograph measures the electrical signals associated with the brain. Such instruments have now become relatively common, and most hospitals have facilities wherein the electrical signals from the bodies of patients can be measured to determine certain types of possible disease or abnormality.
More recently, medical researchers have discovered that the body produces magnetic fields naturally or when properly stimulated, of a type completely different from the other types of energy emitted from the body. The research on correlating magnetic fields and responses with various states of health, disease and abnormality is underway. It has been demonstrated, among other things, that deficiencies or excesses of iron in the body can be determined quantitatively by the paramagnetic response of iron-containing molecules in the liver.
The normal, healthy human body typically contains about 60 milligrams of iron per kilogram of body weight (or about four grams of iron in a typical adult male). A large deficiency or excess of iron in the body can be clinically significant. A deficiency of iron deplete bodily reserves, interfere with hemoglobin production, and lead to anemia in severe cases. An excess of iron can indicate shifts in body chemistry or disease, such as hereditary hemochromatosis, in the early stages of refractory anemias and sometimes in liver disease.
Early diagnosis of iron deficiency or excess imbalances in the body is particularly important, as these problems can often be effectively treated at an early stage. Several techniques have been developed for determining the iron content of the body. Indirect methods involve measurements of the levels of chemicals whose presence and amount are thought to be related to iron level in the body. These methods, such as measurement of serum ferritin or urinary iron excretion, are not sufficiently quantitative to be useful in detecting the early stages of an imbalance. Direct invasive techniques, such as tissue biopsy of the liver, are more quantitative and accurate, but the discomfort and risks associated with their use limit their applicability in screening patients for early indications of iron imbalance.
It has now become possible to make measurements of the iron content of organs, and particularly the liver where iron reserves are stored, by direct magnetic measurements that are noninvasive and therefore particularly suitable for early diagnosis. A biomagnetic susceptometer is an instrument having a magnetic excitation coil which excites a paramagnetic response in iron-containing molecules in the body, and having a very sensitive magnetic detector to measure the paramagnetic response. The biomagnetic susceptometer is placed near to the body of the patient, and the patient's iron levels are measured without any known ill effects on the patient. The patient is unaware of any sensation of measurement, except that he is moved cyclically toward and away from the measurement instrument.
Biomagnetic susceptibility measurements require extraordinarily sensitive and sophisticated magnetic detectors and techniques for avoiding spurious noise signals. The fields to be measured from the liver are typically less than 1/100,000 as great as the magnetic field of the earth in which the instrument and patient are immersed. Nearby electrical equipment, metals, implants, and even the signals from other organs of the body can interfere with the signal obtained from the organ under study.
At the heart of the biomagnetic susceptometer are specialized magnetic field sensing coils, and detectors called Superconducting QUantum Interference Devices (or "SQUIDs"). These devices, which measure very small magnetic signals, operate in the superconducting temperature range for their materials of construction. In the current approach common to most types of superconducting apparatus, the superconducting temperature is achieved with a bath of a cryogenic fluid which is maintained as a liquid but boils to remove heat during operation. The SQUIDS are placed into the cryogenic fluid bath for stabilized operation at the required temperature.
In an existing biomagnetic susceptibility measurement instrument, the magnetic field sensing coils, magnetic excitation coils, and SQUIDs are immersed in a container of liquid helium at a temperature of 4.2.degree. K (i.e., near to absolute zero). The field sensing coils and magnetic excitation coil are placed near the bottom of the container, within a few centimeters of the patient. The container, insulation, and related components are made of special materials that do not interfere with the magnetic measurements. For example, the container itself is made of a fiberglass that has substantially no magnetic susceptibility. The magnetic excitation coil is operated to excite a paramagnetic response in the iron-containing molecules in the patient's liver, and the response is detected by the magnetic field sensing coil and the SQUID working together. The instrumentation is designed to minimize interference from magnetic signals, both steady and varying, other than those for which a measurement is sought.
The existing biomagnetic susceptibility instruments have been shown to give measurements of iron concentration in the liver that correlate very well with measurements made by biopsy or other invasive technique, particularly for conditions of excess iron. However, there is some lack of resolution of the iron content, particularly for iron levels below normal. In these cases, the paramagnetic signal may be masked by spurious fields and influences, and measurement becomes difficult. Complex, expensive electronics can be used to resolve the small signal for the background, with reasonable effectiveness. Magnetically quiet enclosures are also used to reduce the background noise and thence improve resolution of the paramagnetic response of the liver. Nevertheless, there continue to be limits to the resolution possible with existing biomagnetic susceptometers, and it would be desirable to improve the ability of the instruments to detect weak signals.
Thus, there exists a need for an improved apparatus for measuring small biomagnetic responses induced by an external magnetic signal. Such improved technology would also be of value in other areas where weak magnetic responses are studied, such as geology and marine studies. The present invention fulfills this nedd, and further provides related advantages.